Exhaust manifolds were long manufactured by casting. Thereafter, as needs for energy saving and emission control increased, the conventional manufacturing method has come to be replaced by the pressing and welding of metal pipes and plates, which are lighter in weight and more heat-resistant than castings to manufacture the manifolds. In particular, exhaust manifolds produced by the new method are widely used for sports cars and similar cars equipped with superchargers.
While having the two good points, namely, the lightness of the material in weight and its greater resistance to heat, the new manufacturing method however has the following drawbacks:
(a) The metal to be made into the manifold may be deformed (by welding operation) where it is welded to a pipe and, for this reason, it may be that a flange coupling of small thickness cannot be used. PA1 (b) The new manufacturing method requires a great deal of skill in welding operation. PA1 (c) Exhaust manifolds produced by the new method may have different qualities, particularly due to differences in skill at welding operation. PA1 (d) The heat used for welding operation deteriorates the material for the manifold. PA1 (e) Major requisites for welding operation, including the welding apparatus, electric power, welding electrode, and welding gas, cost much. PA1 (f) Welding operation pollutes its environment.